Prior to the past several decades, scientists generally believed that it was impossible to restore hearing to the profoundly deaf. However, scientists have had increasing success in restoring normal hearing to the deaf through electrical stimulation of the auditory nerve. The initial attempts to restore hearing were not very successful, as patients were unable to understand speech. However, as scientists developed different techniques for delivering electrical stimuli to the auditory nerve, the auditory sensations elicited by electrical stimulation gradually came closer to sounding more like normal speech. The electrical stimulation is implemented through a prosthetic device, known as a cochlear implant (CI), which is implanted in the inner ear.
Cochlear implants generally employ an electrode array that is inserted into the cochlear duct. One or more electrodes of the array selectively stimulate different auditory nerves at different places in the cochlea based on the pitch of a received sound signal. Within the cochlea, there are two main cues that convey “pitch” (frequency) information to the patient. There are (1) the place or location of stimulation along the length of a cochlear duct and (2) the temporal structure of the stimulating waveform. In the cochlea, sound frequencies are mapped to a “place” in the cochlea, generally from low to high sound frequencies mapped from the apical to basilar direction. The electrode array is fitted to the patient to arrive at a mapping scheme such that electrodes near the base of the cochlea are stimulated with high frequency signals, while electrodes near the apex are stimulated with low frequency signals.
A sound coding strategy is an algorithm that translates signals picked up by a microphone into a sequence of electric pulses that can be transmitted to the intra-cochlear electrodes. While existing sound coding strategies can support satisfactory recognition of speech in quiet environments, subjects' speech comprehension can decrease dramatically in difficult listening conditions. Some causes of poor performance in demanding listening tasks may be partially attributed to sound processing strategies themselves. For example, all strategies assume that each channel is represented independently in the cochlea. However, the performance of CI users can generally be worse than that of normal-hearing subjects listening through cochlear implant simulators with a similar number of channels. This suggests that a substantial amount of channel interaction can occur in cochlear implant subjects. Channel interaction may “smear” spectral peaks that are essential, for example, for encoding vowel identity. Thus, performance of some CI subjects might be improved by utilizing a strategy that emphasizes peaks in short-term spectra.
Accordingly, U.S. Pat. No. 7,242,985, incorporated herein in its entirety by reference, discloses a cochlear stimulation system that accounts for the interaction between frequency bands and thereby enhances the contrast between neighboring signals. The cochlear stimulation system implements an outer hair cell model strategy in which lateral suppression coefficients are adjusted to distinguish the contributions of individual signals to the composite signal defining a sound. In general, this is accomplished by dividing an audio signal into multiple input signals such that each input signal is associated with a particular frequency band. Each of the input signals can then be scaled in accordance with respective scaling factors representing the separation between different frequency bands. Once scaled, the multiple input signals can be converted into simulation signals that represent the laterally suppressed sound.
Another contrast enhancement strategy, the Moving Picture Experts Group MP3 audio layer 3 (MP3) strategy, is based on a psychoacoustic model which recognizes that there are certain sounds that a human ear cannot hear. Psychoacoustics describes a relationship between physics of sounds and their perception. An absolute hearing threshold and masking are two fundamental phenomena in psychoacoustics. The absolute hearing threshold is the minimum intensity for the human ear to detect sound at a given frequency in quiet. Masking describes the nonlinearly raised threshold for the human ear to detect sound in the presence of an existing masking signal. The human ear has a naturally masking function. If two sounds are very different but one is much louder than the other, the human ear may not perceive the quieter signal. While hearing capacity varies from person to person, in general, the human ear cannot hear outside the 20 Hz to 20 kHz range. In addition, the human ear is more sensitive to certain frequencies, typically between 2 kHz to 4 kHz. The MP3 strategy takes advantage of the naturally masking function of the human ear to mask sound information that a normal human ear can not perceive.
The MP3 strategy implements multiple perceptual codecs to eliminate the sound information inaudible to the human ear. A perceptual codec can be subdivided into multiple discrete tasks. A sound signal can be broken into smaller component pieces called frames, each with a short (fraction of a second) duration. Also, the spectral energy distribution, frequency spread, can be determined by breaking the signal into multiple sub-bands and processing them independently. The frequency spread for each frame can be compared to mathematical models of human psychoacoustics stored in the codec to determine which frequencies need to be rendered accurately. All other frequencies that cannot be perceived by the human ear are removed through masking effects.
While the outer hair cell model and the MP3 strategy provide effective sound processing strategies, neither technique provides a customized contrast enhancement strategy based on the specific needs of an individual CI user. Since no people experience exactly the same hearing loss, there exists a need to provide optimal spectral contrast enhancement tailored to the individual cochlear implant user. The patients who experience a small amount of smearing would perceive spectral contrast enhancement as a distortion of sound, since the smaller spectral peaks would be suppressed. On the other hand, patients who experience a large amount of spectral smearing, would benefit from spectral contrast enhancement. This is because spectral contrast enhancement allows large spectral peaks to be heard, while suppressing the neighboring ones. For patients with a large amount of smearing, neighboring channels introduce noise to the patient, and if suppressed, this noise can be decreased. Therefore to create the optimal amount of spectral enhancement, it is desirable to find out how much spectral smearing is present for the individual patient.